1. Field of the Invention
This invention relates to hydrocarbon-soluble coordination compounds of divalent manganese, iron, cobalt, and nickel which are useful as antiknock additives in fuels.
2. Description of the Prior Art
One method for improving gasoline antiknock quality (also known as octane quality) is by increasing therein the content of high octane hydrocarbons such as benzene, toluene and the like. This use of high octane hydrocarbon components is relatively uneconomical since they are more valuable as solvents and chemical feedstocks than as gasoline components.
Another method for improving the antiknock quality of gasoline is to add antiknock additives. In the past, tetraalkyllead compounds have been the most popular antiknock additives. The advent of automobiles equipped with catalytic converters has brought the attendant requirement for lead-free gasoline. Thus, there is the corresponding need to develop acceptable antiknock additives to replace those containing lead.
Most lead-free antiknocks suggested in the art are commercially unacceptable because of one or more deficiencies such as high cost, low antiknock activity, hydrolytic-, thermal-, or oxidative-instability, insufficient solubility in gasoline, inadequate volatility, and too-high water solubility. Before discussing the compounds of this invention, a brief summary of some of the prior art teachings will be provided.
British Patent No. 287,192 teaches that .beta.-diketone (such as acetylacetone) compounds of heavy metals such as Fe, Ni, Co, Cr, Th, Cu, Mn, Mo, V and W are useful antiknocks for hydrocarbon fuels. However, in spite of suggestions in the art of processes of stabilizing and solubilizing metal .beta.-diketonates in hydrocarbon fuels, e.g., U.S. Pat. Nos. 2,144,654 and 2,156,918, the metal .beta.-diketonates have not been used commercially as antiknocks.
The utility of the above-mentioned metal .beta.-diketonates as gasoline antiknocks is limited by their low solubility in gasolines and their low volatility. The divalent cobalt compound of acetylacetone, bis(acetylacetonate)Co(II), has a solubility in gasoline of less than 0.025% at room temperature.
Recent studies have indicated that the acetylacetonates of Mn, Fe, Co, and Ni are polymerized (trimers and tetramers) in the solid state and in nonpolar solvents. The oligomeric nature of these compounds most probably explains their low solubility in gasolines and their low volatility. See Graddon, Nature, 195, 891 (1961), "Polymerization of Transition Metal .beta.-Diketone Chelates"; Cotton et al, J. Am. Chem. Soc., 86, 2294 (1964) "The Tetrameric Structure of Anhydrous Crystalline Cobalt (II) Acetylacetonate"; and Graddon, Coordin. Chem. Rev., 4, 1 (1969), "Divalent Transition Metal .beta.-Keto-Enolate Complexes as Lewis Acids".
From the above, it might appear that a transition metal chelate of .beta.-diketones which would be monomeric in hydrocarbons such as gasoline would have the required solubility, volatility, and antiknock activity. It is known to prepare divalent transition metal chelates of .beta.-diketones which are monomeric in the solid state and in nonpolar solvents by using a .beta.-diketone with bulky groups to preclude self-polymerization of the chelate compounds. Thus, chelate compounds of divalent transition metals with 2,2,6,6-tetramethyl-3,5-heptanedione (dipivaloylmethane, DPM) are reported to be monomeric in the solid state and in non-polar solvents. However, such compounds are not practical as antiknock additives because of their great sensitivity to air oxidation. Fe(DPM).sub.2, for example, is reported to "char immediately on exposure to air"; see Fackler et al, Inorg. Chem., 6, 921 (1965). Mn(DPM).sub.2 "charred immediately on contact with air"; see Hammond et al, Inorg. Chem., 2, 75 (1963). Extreme sensitivity of Co(DPM).sub.2 to oxidation is noted in the Hammond reference supra, p. 76, and by Gerlach et al in Inorg. Chem., 8, pp. 2293 and 2294 (1969).
Metal chelates with .beta.-diketones containing substituents such as fluorine to enhance the volatility of the metal chelates have also been prepared. Haszeldine et al, J. Chem. Soc. 609 (1951), prepared uranium chelates of trifluoroacetylacetone (TFAA) and hexafluoroacetylacetone (HFAA) which could be sublimed without decomposition. Morris et al, Inorg. Chem. 2 411 (1963), prepared dihydrates of hexafluoroacetylacetonates of zinc, nickel, cobalt, manganese and iron, but attempts to dehydrate these complexes by sublimation were unsuccessful.
Considerable work has also been done with copper chelates of fluorinated .beta.-diketones primarily because of the ease of formation of the chelate compounds which are not subject to oxidation as are those of Co.sup.++ and Fe.sup.++. See, for example, Calvin et al J. Am. Chem. Soc. 67, 2003 (1945); Reichert et al, Canadian J. Chem. 48, 1362 (1970); Fenton et al J. Chem. Soc. 1577 (1971); and McMillin et al, J. Am. Chem. Soc. 98 3120 (1976).
Fenton et al, in J. Chem. Soc. 1577 (1971) disclose complexes of bis(hexafluoroacetylacetonate)Copper II with some nitrogen bases and glycols. Among the bidentate nitrogen bases, are disclosed ethylenediamine, N,N'- and N,N-dimethylethylenediamine, N,N,N',N'-tetramethylethylenediamine (TMED) and N,N,N',N'-tetramethyl-o-phenylenediamine. Izumi et al, in Bull. Chem. Soc. (Japan) 48 3188 (1975) disclose nitrogen base addition products of bis(hexafluoroacetylacetonato) complexes of Co(II), Ni(II), Cu(II) and Zn(II). The two disclosed bidentate nitrogen bases are 1,10-phenanthroline and 2,2'-bipyridine.
The antiknock activity of the compounds described herein, particularly that of the preferred cobalt compound, is unexpected in view of the teaching in the art that cobalt compounds are not considered to be good antiknocks. Thus, a paper presented at the American Petroleum Institute, Division of Refining, May 14, 1971 (API preprint No. 47-71 at page 859) by Unzelman et al, stated that cobalt compounds are mild antiknocks and that the metal compounds having maximum effectiveness are those with metal atoms bonded to carbon.
On the contrary, the compounds described herein have no metal to carbon bonds and yet they provide effective antiknock performance. In fact, a preferred cobalt compound, as will be detailed more completely in Example 32, is more efficient by about 130% to 240% on a metal weight basis versus a manganese compound of metal to carbon bonds and proven antiknock performance.